It is known that the atomic nucleus containing an odd number of protons or an odd number of neutrons has magnetic and hence generates a nuclear magnetic dipole moment. When this nuclear magnetic dipole moment is placed in a static magnetic field, the nuclear magnetic dipole moment makes a rotary motion called precession at an angular frequency determined by the product of the intrinsic magnetogyric ratio of the nucleus and the intensity of the static magnetic field. In this state, when a rotating magnetic field is applied to the nuclear magnetic dipole moment at this angular frequency, the precession of the nuclear magnetic dipole moment becomes gradually violent.
Regarding this motion in a rotating coordinate systems rotating at the same angular frequency as in the rotating magnetic field, putting the z' axis in the direction of static magnetic field, the nuclear magnetic dipole moment is tilted from the z' axis toward the x'-y' plane. This tilting angle generally is called the flip angle, which is determined by the magnetogyric ratio, intensity of rotating magnetic field, and application time of the rotating magnetic field.
When the rotating magnetic field is applied in the condition of the flip angle of, for example, 90.degree., and then the rotating magnetic field is stopped, the nuclear magnetic dipole moment returns gradually from the x'-y' plane to the initial state aligned in the z' axis before application of the rotating magnetic field while making precession. This process is explained by two processes, that is, longitudinal relaxation process for recovering the magnetized components in the static magnetic field direction, and transverse relaxation process for attenuating the magnetized components in the x'-y' plane. The longitudinal relaxation process is also called the spin-lattice relaxation or T1 relaxation, and its time constant is generally expressed as T1. The transverse relaxation process is called the spin-spin relaxation or T2 relaxation, and its time constant is T2.
Such magnetizing motion in the relaxation process can be observed by a coil placed in the x'-y' plane. In other words, the precession of magnetization induces a magnetic flux change in the coil, and therefore, according to the Faraday's electromagnetic induction law, an electromotive force at the aforementioned angular frequency is generated at both ends of the coil. This electromotive force is the so-called magnetic resonance signal (MR signal).
From the MR signal collected by utilizing such nuclear magnetic resonance phenomenon, the spatial distribution of a specific nucleus and the state of various molecules containing the nucleus can be observed.
To obtain a magnetic resonance image of high diagnostic ability, it is important to detect the resonance frequency of the nucleus accurately and enhance the spatial uniformity of static magnetic field.
Herein, to achieve the two objects, hitherto, as the preparatory steps for imaging, the processes called frequency lock and shimming are executed.
The frequency lock is to specify the resonance frequency or center frequency of the nuclide (since the resonance frequency has a certain bandwidth, its central value (or peak value) is thus called representatively). For instance, if the intensity of the static magnetic field is 1.5 T, it may be actually 1.5 T.+-..alpha. owing to the drift of the main magnet or other factors, and when the intensity of the static magnetic field is deviated from 1.5 T, the resonance frequency of the nuclide is deviated accordingly, and hence frequency lock is an indispensable step. Without this step, the area may be different from the region to be imaged, and other spin than aimed may be excited.
Shimming or active shimming is to correct spatial fluctuation of static magnetic field (nonuniformity of static magnetic field). Nonuniformity of static magnetic field occurs also in the presence of the object, and if it is not corrected, various artifacts may be caused or undesired region may be excited. In a typical example, therefore, the spatial magnetic field distribution of the imaging region is determined, and the linear or nonlinear gradient magnetic field for making it uniform is obtained, and shimming is achieved by superposing it on the static magnetic field.
In the same nucleus, however, the intensity of the magnetic field that the nucleus actually feels varies somewhat due to the effect of electrons surrounding the nucleus, that is, the magnetic field shielding effect. Since the state of electrons varies with the molecules containing the nucleus, and therefore the resonance frequency is slightly deviated depending on the molecules. This deviation is called the chemical shift. For example, supposing the object nucleus to be proton (.sup.1 H), the difference in chemical shift between water and fat containing it is deviated by about 3.5 ppm as shown in FIG. 7. This 3.5 ppm corresponds to about 224 Hz, supposing the static magnetic field intensity to be 1.5 tesla.
This chemical shift includes various artifacts, for example, deviation of position of water and position of fat on the image, but, to the contrary, it is attempted to utilize this chemical shift. Principal applications include the chemical shift imaging for providing the image only for the nucleus contained in a specific molecule, and MR spectroscopy for presenting frequency spectrum of MR signal. From these data, for example, a water image may be created, or various useful information relating to metabolic functions, for example, the mode of compound produced by metabolism can be obtained.
Such chemical shift imaging and MR spectroscopy are significantly influenced by the non-uniformity of the static magnetic field. In particular, the chemical shift selective pulse (CHESS pulse) widely used in such chemical shift imaging or MR spectroscopy is likely to be influenced by the non-uniformity of static magnetic field. This CHESS pulse is adjusted in a relatively narrow bandwidth around the actual resonance frequency so as to excite or invert selectively only the specific nucleus contained in the specific molecule. For effective function of the CHESS pulse, the so-called frequency lock is indispensable, that is, the center frequency must be accurately adjusted to the resonance frequency of the specific nucleus contained in the specific molecule. Also, in order that the action the magnetization receives from the CHESS pulse may not fluctuate depending on the location, the current flowing in the shim coil must be adjusted to suppress spatial variations of the strength of static magnetic field, that is, non-uniformity of static magnetic field at, for example, less than 1 ppm, and hence the magnetic field distribution is dynamically corrected, which is known as dynamic shimming.
For frequency lock, the actually collected MR signal is Fourier transformed, and from the obtained frequency spectrum, it is required to measure accurately the true value of the resonance frequency of the object nucleus. FIG. 1 shows a frequency spectrum obtained by Fourier transform of the MR signal collected from the proton spins. Since the resonance frequency Ffat of proton contained in the fat molecule is lower than the resonance frequency Fwater of proton contained in the water molecule, the lower one of the two peaks can be identified as Ffat and the higher one as Fwater.
However, if one of fat and water is hardly present within the object slice, only one peak appears in the frequency spectrum. Therefore, only two peak frequencies cannot be judged to be either Ffat or Fwater.
The effect of shimming is exhibited only when performed along the accurate magnetic field distribution. Therefore, the difference in chemical shift between fat and water is a serious obstacle for accurate measurement of magnetic field distribution.